1. Field of the lnvention
This invention pertains to methods and means for the production of urea fluorosiiicate, a chemical compound having the formula [(NH.sub.2).sub.2 CO].sub.4.H.sub.2 SiF.sub.6, by new and improved processes employing, for the feedstock thereto, urea and fluorosilicic acid, said fluorosilicic acid being a by-product of the phosphate fertilizer industry. The mode of operation of the instant invention involves (1) reaction of urea and aqueous fluorosilicic acid; (2) concentration of the resulting aqueous solution of urea fluorosilicic acid; and, if desired, (3) crystallization of the product urea fluorosilicate as a solid. 2. Description of the Prior Art
Numerous prior art investigators have discovered, taught, and disclosed a number of me(hods for trying to overcome, or otherwise circumvent the problems associated with widespread occurrence ofwwheat stem rust, including improving upon the process for the production of urea fluorosilicate. For instance, according to the teachings of Zeying Zhang et al. [Hua Hsueh Tung Pao. 1981, (3) 142-143; Kexue Tongbao 1982, 27 (11, 658-62; Kexue Tongbao, 1983, 28 (7), 905-910 ] urea flourosilicate having the formula [(NH.sub.2).sub.2 CO].sub.4.H.sub.2 SiF.sub.6 is a highly effective and practical agent for prevention and control of wheat stem rust. The compound is highly soluble and its concentrated solutions are relatively noncorrosive in comparison to those of concentrated fluorosilicic acid. The production and use of urea fluorosilicate has been extremely limited because heretofore no practical process was or has been available for its economical production. The only known method for its preparation, used by Zhang et al. supra, is based on the following equation EQU 4(NH.sub.2).sub.2 CO+2SiF.sub.4 +4CH.sub.3 OH=[(NH.sub.2).sub.2 CO].sub.4.H.sub.2 SiF.sub.6 +2HF+Si(OCH.sub.3).sub.4
and requires the use of methyl alcohol and silicon tetrafluoride as raw materials. The yield of product is only 50 percent, based on silicon fluoride, even under the best of operating conditions. Furthermore, large amounts of by-products, namely hydrogen fluoride and methoxysilane, are produced which require additional processing, recovery, and disposal.
Other investigators have studied the solubility of urea in hexafluorosilicic acid [B. A. Beremzhanov. N. N. Nurakhmetov, A. Tashenov, and F. O. Suyundikova, Russian Journal of Inorganic Chemistry. 32 (1), 146, (1987)]. They reported the formation of diurea dihydrogen hexafluorosilicate and tetraurea dihydrogen hexafluorosilicate, but gave no conditions for isolation and purification of these compounds. They repor(ed no chemical analyses, X-ray powder diffraction patterns, refractive indices, infrared absorption data, or other properties to characterize and identify their products. Consequently. their results add but little to the pool of information already existing on urea fluorosilicate, i.e., that shown by Zhang et al. supra.
It is obvious, therefore, that a need exists for the development of an improved process for the practical and economical production of urea fluorosilicate, particularly a process that utilizes low-cost, readily available fluorosilicic acid produced as a by-product in the acidulation of phosphate rock.
Phosphate rock, also known as fluoroapatite, is mined in huge quantities in several countries throughout the world and used in the production of fertilizers, phosphoric acid, and other phosphate compounds. Fluoroapatite usually contains 3 to 4 percent fluorine and a very significant amount of this fluorine is evolved as gaseous effluent in the production of fertilizers. The exhaust gases are ordinarily scrubbed in water so as to obtain a solution of fluorosilicic acid. The resulting scrubber solution usually contains 20 to 28 percent H.sub.2 SiF.sub.6 . Many producers dispose of this potentially valuable fluorosilicate as waste material because of quite limited uses for it. Thus, it is highly desirable, from both an economical and environmental viewpoint, to find new uses for such by-product fluorine resulting from the various process operations practiced in the fertilizer industry.